So my 5900x is pretty toasty...

[Drumroll]

May be worth checking if you can drop voltages. Spent yesterday doing that, and finally got the 3900x (which I guess is as hot) to be considerably cool. Won't be a ready recipe, as every chip, motherboard, RAM, etc will affect your results. Mine is running with PBO on, Offset - 0.2. Boosting Single Core close to 4.7GHz, Cinebench R23, All Core 4.3 GHz, 52C max. Idle low to mid 30s, coolant temperature 26C ATM.
Not comparable as using a custom loop, Monobloc (not sure how less or more efficient is compared to a CPU block), but offset voltage is your best bet. Going - 0.1 or - 0.15, the temperature increased about 5C and maxed out at 4.15/4.2 All core.
Just a bit of patience and you may find the balance for your CPU.

 

[VincentHanna]

Its better now anyway.

Idle in BIOS is 37°C and doesn't really go above 80°C now under heavy load.

That's fine.

 

[Demon_mkII]

May be worth checking if you can drop voltages. Spent yesterday doing that, and finally got the 3900x (which I guess is as hot) to be considerably cool. Won't be a ready recipe, as every chip, motherboard, RAM, etc will affect your results. Mine is running with PBO on, Offset - 0.2. Boosting Single Core close to 4.7GHz, Cinebench R23, All Core 4.3 GHz, 52C max. Idle low to mid 30s, coolant temperature 26C ATM.
Not comparable as using a custom loop, Monobloc (not sure how less or more efficient is compared to a CPU block), but offset voltage is your best bet. Going - 0.1 or - 0.15, the temperature increased about 5C and maxed out at 4.15/4.2 All core.
Just a bit of patience and you may find the balance for your CPU.

3900x runs much cooler on idle, not much different at load from my experience

 

[Wingz]

with Custom Watercooling I get 28-30C idle. Cinebench R20 65C, Realbench around 70C after 15 mins stress test. Playing Read Dead 2 is around 50C.

This is with voltage on Auto and PBO with Curve Optimizer. Cinebench I get all core 4.65ghz Boost. single core boost is 4950Mhz.